This invention relates to fusible sheetform articles, particularly to fusible plastic binding strap that can be joined by friction fusion, hot knife techniques, or the like manner.
Plastic strap is a convenient and relatively inexpensive strapping material that has been used for a wide variety of tying and packaging operations. For many applications plastic strap is uniquely suited by virtue of the inherent elasticity thereof, e.g., for tying packages subject to dimensional change, or to handling situations whereby shock conditions may be imposed upon the strap loop that surrounds the package. Tying usually is accomplished by forming a strap loop about the package, shrinking or reducing the formed loop to a snug fit about the package, and thereafter joining overlapping ends of the strap loop by means of a wrap-around seal or a fused joint.
Wrap-around seals for plastic strap are generally formed in a manner analogous to steel strap, e.g., by crimping a deformable metal band around overlapping strap ends so as to form a mechanical interlock. Such wrap-around seals are not completely effective, however, because plastic strap has inherently low shear strength which restricts the crimping and interlocking techniques normally utilized with wrap-around seals.
As an alternate strap sealing approach, strap joints have been formed by melting and fusing overlapping portions of thermoplastic strap so as to form a joint. For this purpose heated pressure jaws, high frequency dielectric heating means, ultrasonic welders, and friction fusion devices have been used. None of the foregoing joint-forming means are capable of producing routinely and consistently, and in an economic manner, a seal that exhibits a joint strength that is greater than about 40 to 50 percent of the plastic strap tensile strength. It is very desirable, however, to have joint strengths that approach the tensile strength of the strap much more closely.